Metal containing compound reduction and melting process

ABSTRACT

The present invention provides a metal containing compound reduction and melting process which entails feeding a burden made of a mixture of the metal containing compound and a suitable reductant in particulate form into an electrically heatable vessel which contains a bath of the metal in liquid form so that a reaction zone is formed in the burden in which the metal containing compound is reduced and a melting zone is formed below the reaction zone in which the reduced metal is melted; and controlling the process in such a manner that substantially all of the reduction of the metal containing compound takes place in the solid phase. The present invention also provides an apparatus for carrying the process into operation, which has a single compartment channel type induction furnace in which the reduction and melting are carried out, the compartment being provided with at least one feeding port for the burden; at least one exit port for the liquid metal product; and at least one exit port for any slag formed in the reaction.

This application is a national stage of PCT/EP97/01999 filed Apr. 17,1997.

This invention relates to a metal containing compound reduction andmelting process of the type which entails feeding a burden comprising amixture of the metal containing compound and a suitable reductant inparticulate form into the heating vessel of a channel type inductionfurnace which contains a bath of the said metal in liquid form so thatat least one heap of burden forms above the metal bath.

Such process is known from U.S. Pat. No. 5,411,570.

Most conventional metal containing compound reduction processes involvethe heating of the metal containing compound (usually the metal oxide)in the presence of a reductant such as a suitable carbon containingcompound or the like. The metal containing compound and reductant areusually collectively referred to as the burden.

In the aforesaid conventional processes, the rate at which such heatingtakes place is usually so rapid that at least a substantial part of theburden is melted before the reduction reactions are completed. Asubstantial amount of the reduction accordingly takes place in theliquid phase.

On such melting of the burden, a slag is formed which, apart from theoxides of the said metal containing compound, also contains the oxidesof other metals which may be present in the burden.

In order to recover the said metal containing compound from its oxide inthe slag, a further amount of the reductant is required in the reactionwhich can accordingly give rise to a metal product with an unwanted highcarbon content.

Such high carbon in the product is usually subsequently reduced byoxidation, either by adding to the product oxygen as gaseous oxygen orair, or by utilising a suitable metal oxide present in the reactionmedium. Such oxidation reaction accordingly also takes place in theliquid phase.

It will be appreciated that apart from requiring an initial excess ofreductant, the subsequent oxidation of such excess entails a furtherstep in the process.

As a result of both the aforesaid reduction and oxidation reactions,relatively large volumes of unwanted gas are formed below the surface ofthe liquid metal and slag which then escape in the form of bubbles fromthe liquid metal and slag.

In above-mentioned U.S. Pat. No. 5,411,570 a steel making process isdisclosed in which the burden is added to the furnace in two heaps whichremain separated from each other by a slag layer, floating on top of theliquid metal bath. In this arrangement it is possible for the burden topass directly into the metal bath or any slag which may be present, sothat at least part of the reduction of the burden takes place in theliquid phase, which not only gives rise to unwanted gas formation in thefurnace but also to a potential loss of product.

It is accordingly an object of this invention to provide a metalreduction and melting process with which the aforesaid problems can beovercome or at least minimised.

According to the invention this object is achieved by a process of thetype described, in which the burden is fed into the vessel in such amanner and rate that it forms a continuous layer of burden which extendsin the manner of a bridge over the whole of the liquid bath and any slagwhich may be present, with the result that a reaction zone is formed inthe burden in which all the metal containing compound can be reduced,and a melting zone is formed below the metal reduction zone in which allthe reduced metal can be melted; and controlling such process in such amanner that said continuous layer is maintained during substantially thewhole duration of the process, and so that all the reduction of themetal containing compound takes place in the solid phase.

It will be appreciated that because substantially no liquid phasereactions take place during the process according to the invention, theaforesaid unwanted gas evolution is practically eliminated, and inpractice the substantial absence of bubble formation in the liquid metalbath and slag which is formed is used as an indication that the processis being controlled correctly.

It will be appreciated further that because the process can be carriedout in such a manner that there is substantially no need for thesubsequently removal of any excess reductant, the number of steps in theprocess is reduced relative to what the case is in the aforesaid knownprocesses.

Further according to the invention the control of the process iseffected by controlling at least one of the following parameters:

(1) the manner in, and rate at, which the burden is fed into the vessel;

(2) the particle size of the burden;

(3) the degree of mixing of the burden, and

(4) the rate at which heat is supplied to the vessel.

Since according to the invention the burden is fed into the vessel insuch a manner and rate that it forms a continuous layer of burden overthe whole of the liquid bath and any slag which may be present, anyunreacted burden will be prevented from coming directly into contactwith the liquid metal and slag. Such "short circuiting", which couldgive rise to at least some of the reactions taking place in the fluidphase, is accordingly substantially eliminated.

Where the burden is, for example, fed into the vessel through spacedapart feeding ports to form adjacently located heaps of burden insidethe vessel, the process includes the step of ensuring that such feedingis effected in such a manner that the bottoms of the heaps merge todefine the continuous layer of burden, which extends in the manner of abridge over the liquid bath and slag. Such bridge accordingly preventsburden material falling from the heaps from coming directly into contactwith the liquid metal or slag.

The fact that such a bridge is being formed can be established in anysuitable manner such as, for example, visually, and/or by means of imagerecording apparatus, such as cameras, etc. In practice, such visualrecording may be effected by inserting a rigid element in the manner ofa "dip stick" from the top of the vessel into the burden.

The formation of the said bridge may be effected by controlling the sizeof the burden heaps inside the vessel, or alternatively and/oradditionally, it may be done by the strategic location of the portsthrough which the burden is fed into the vessel and/or by controllingthe number of such ports and the rate at which the burden is fed throughthem.

Further according to the invention the particle size of the burden is sochosen that it can pass through a 10 mm, preferably 6 mm, morepreferably 3 mm, sieve.

Applicant has found that when a burden of such small particle size isemployed, substantially the whole of each particle is reduced to theparticular metal in the reaction zone and accordingly remains solidbefore the temperature of the particle is elevated to that required forthe melting of unreduced oxides which may be present in the particle.

There is accordingly very little tendency for any liquid metal in theform of the metal oxide melting at a lower temperature than the metal,to escape from such a particle into the slag.

Thus, for example, in the case of iron, the nucleus of the particleusually comprises FeO, which melts at a temperature of 1378° C., whilethe crust of the particle comprises Fe which only melts at 1535° C.Accordingly, if larger particles than those stipulated above areemployed, the temperature of the nucleus of such a particle could beraised to the aforesaid temperature of 1378° C. before all the Fe or FeOin the particle is reduced, which may lead to the liquid FeO escapingfrom the nucleus.

It will be appreciated that because the said solid phase reactions arediffusion controlled, the maximum rate of heat input which will berequired will be a function of the particle size and degree of mixing ofthe components of the burden, such degree of mixing preferably beingsuch that the burden comprises a homogeneous mixture.

Further according to the invention the process may include the step ofburning above the burden the CO which is formed during the reduction ofthe metal containing compound, and which permeates through the burden.

Such burning may, for example, be effected by suitable oxygen and/or airburners located in the vessel above the burden.

It will be appreciated that the heat so produced will also serve toincrease the temperature inside the vessel, mainly through radiationfrom the roof of the vessel.

Preferably the reaction vessel comprises the heating compartment of achannel type induction furnace.

Applicant has found such an arrangement particularly suited because ofthe ease with which the rate of heating can be controlled in such afurnace.

Further according to the invention an apparatus for carrying theaforesaid process into operation comprises a single compartment channeltype induction furnace in which the said reduction and melting arecarried out, the compartment being provided with at least one feedingport for the burden and at least one exit port for the liquid metalproduct; and/or any slag formed in the reaction, said at least onefeeding port being so located and of such a size that the burdenintroduced through it can extend as a continuous layer over the whole ofthe liquid metal bath and any slag which may be present in thecompartment.

In a preferred form of the invention, the metal containing compoundpreferably comprises or includes an iron containing compound. In otherforms of the invention the metal of the metal containing compound maycomprise or include any other suitable one such as chromium and/ormanganese and/or copper and/or zinc and/or lead, etc.

One embodiment of the invention will now be described by way of examplewith reference to the enclosed drawing, which is a diagrammatic crosssectional view through a furnace according to the invention.

BRIEF DESCRIPTION OF THE DRAWING

In this embodiment of the invention a channel type induction furnace 10is utilised which comprises an elongated tubular compartment 11 ofcircular configuration in cross section which is provided along itsbottom with two rows of electrically operated inductors 12, each rowcomprising five such inductors of a capacity in the order of 2.2 MWeach.

Compartment 11 includes two parallel extending rows of feeding ports ofwhich only two, 13 and 14, are shown, which extend along oppositelongitudinal sides of compartment 11. These ports are utilised forintroducing a burden 15 to furnace 10 to form two longitudinallyextending heaps 16 and 17 which float on a liquid metal bath 18. Ifrequired, the latter can initially be introduced to vessel 11 through afeeding port, not shown.

Burden 15 comprises a homogeneous mixture in particulate form of acarbon containing compound such as coal, for example, and iron oxide;the carbon containing compound being present in a concentration slightlyless than that representing the stoichiometric amount of carbonnecessary for reducing the ore; and the particle size of burden 15 beingsuch that it can pass through a 3 mm sieve.

Burden 15 is introduced to vessel 11 in such a manner and at such a ratethat the bottoms of heaps 16 and 17 merge with each other to form abridge 19 of burden material which extends over liquid bath 18.

The fact that the bridge 19 is being formed can, for example, beestablished visually by means of a `dip stick` which is inserted fromabove into vessel 11, or by means of a suitable inspection window (notshown) in the wall of vessel 11. It may also be established by means ofa suitable image recording apparatus (also not shown) located insidevessel 11.

Vessel 11 is also provided along its upper end with a plurality ofoxygen burners of which only two, 20 and 21, are shown, by means ofwhich the CO formed in the reaction, and which permeates through theupper layer of burden 15, can be burnt.

In operation, a reaction zone is created in burden 15 of heaps 16 and 17which extends virtually from the bottoms of the heaps to their upperends. At the same time a melting zone 22 is formed which extends betweenthe bottoms of heaps 16 and 17 and the upper surface of liquid bath 18.During the reaction the reduced burden 15 moves under the influence ofgravity from the reaction zone towards melting zone 22.

The slag which is formed during such melting floats on top of bath 18 ina tunnel 23 which also extends below melting zone 22. Tunnel 23 leads toa slag exit port (not shown) in vessel 11, and burden feed ports 13 and14 are so arranged relative to such slag exit port that the slag intunnel 23 is directed towards it.

During the operation of the process, bridge 19 serves to prevent anyburden material 15 from falling directly from heaps 16 and 17 into theslag in tunnel 23, or into the liquid metal in bath 18, thus preventingany `short circuiting`.

The heat supplied to bath 18 through inductors 12 diffuses into burden15 in heaps 16 and 17 and this, together with the heat from the CO beingburnt by burners 20 and 21, causes the iron oxide and carbon of burden15 to react, which results in the reduction of the iron oxide. Almostall of such reduction, which accordingly takes place in the solid phase,takes place in the uppermost 20 mm layer of heaps 16 and 17, mainly dueto the heat being supplied to such layer from the burning of the CO byburners 20 and 21. At the same time the solid reduced iron is melted inzone 22, from where it passes under gravity into bath 18.

It will be appreciated that apart from overcoming the problems referredto in the preamble of this specification as being encountered with theknown arrangements, a further advantage of the process according to theinvention is that because it can operate with a burden of such smallparticle size, burdens can be used which normally would not be usableotherwise than through prior pelletization and/or sintering.

It will appreciated further that there are no doubt many variations indetail possible with a metal containing compound reduction and meltingprocess according to the invention, and apparatus utilised for carryingout such process, without departing from the spirit and/or scope of theappended claims.

What is claimed is:
 1. A metal containing compound reduction and meltingprocess comprisingfeeding a burden comprising a mixture of the metalcontaining compound and a suitable reductant in particulate form into aheating vessel of a channel induction furnace which contains a bath ofthe said metal in liquid form so that at least one heap of burden formsabove the metal bath, characterized in that the burden is fed into thevessel in such a manner and rate that it forms a continuous layer ofburden which extends in the manner of a bridge over the whole of theliquid bath and any slag which may be present, with the result that areaction zone is formed in the burden in which all the metal containingcompound is reduced, and a melting zone is formed below the metalreduction zone in which all the reduced metal is melted; and controllingsuch process in such a manner that said continuous layer is maintainedduring substantially the whole duration of the process, and so that allthe reduction of the metal containing compound takes place in the solidphase.
 2. The process of claim 1 wherein the burden is fed into thevessel though spaced apart feeding ports to form adjacently locatedheaps of burden inside the vessel, of which the bottoms merge to definesaid continuous layer of burden.
 3. The process of claim 1 wherein theformation of the continuous layer is effected by controlling the size ofthe burden heaps inside the vessel.
 4. The process of claim 1 whereinthe formation of the continuous layer is effected by locating portsthrough which the burden is fed into the vessel, by controlling thenumber of such ports and the rate at which the burden, is fed throughthem, or by controlling the level of the liquid metal in the vessel. 5.The process of claim 1 wherein the said control of the process iseffected by controlling at least one of the following parameters:(1) themanner in, and rate at, which the burden is fed into the vessel; (2) theparticle size of the burden; (3) the degree of mixing of the burden; (4)the rate at which heat is supplied to the vessel.
 6. The process ofclaim 1 wherein the particle size of the burden is such that it passesthrough a 10 mm sieve.
 7. The process of claim 6 wherein the particlesize of the burden is such that it passes through a 6 mm sieve.
 8. Theprocess of claim 7 wherein the particle size of the burden is such thatit passes through a 3 mm sieve.
 9. The process of claim 1, wherein CO isformed during reduction of the metal containing compound and permeatesthrough the burden or above the burden, the process further comprisingthe steps ofburning the CO and utilizing heat produced in the reaction.10. The process of claim 9 wherein such burning is effected by oxygenand/or air burners located in the vessel above the burden.
 11. Theprocess of any one of the preceding claims wherein the metal of themetal containing compound comprises or includes any one or more thefollowing: iron, chromium, manganese, copper, zinc, and lead.
 12. Theprocess of claim 1 in which the metal of the metal containing compoundcomprises iron.
 13. Apparatus for carrying out the process of claim 1which comprises a single compartment channel induction furnace in whichthe said reduction and melting are carried out, the compartment beingprovided with at least one feeding port for the burden; and at least oneexit port for the liquid metal product and/or any slag formed in thereaction, characterized in that the at least one feeding port is solocated and of such a size that the burden introduced through it extendsas a continuous layer over the whole of the liquid metal bath and anyslag present in the compartment.